1. Technical Field
Example embodiments of the present invention relate to over-the-air (OTA) inter-cell interference coordination (ICIC) methods in cellular systems, for example, femto-cells (home evolved Node-Bs (eNBs)).
2. Related Art
Cellular systems need to effectively control inter-cell interference so as to improve the throughput of a system. In particular, users located in a cell edge may not secure a sufficient data transmission rate due to interference from a neighboring cell.
To increase the data transmission rate for the users located in the cell edge (hereinafter, referred to as the cell-edge users), an ICIC technique is used in long-term evolution (LTE). In cells upon scheduling, power is allocated to specific radio resources so that inter-cell interference may be reduced through coordination with neighboring cells. In general, this is called ICIC.
For the ICIC, communication for exchanging information for coordination with neighboring cells upon scheduling is necessary. If the neighboring cell is located at the same eNB, information may be exchanged without separate signaling. However, if the neighboring cell is controlled by other eNBs, separate signaling is necessary.
In an LTE downlink (DL), information of relative narrowband transmit power (RNTP) is exchanged between the eNBs through an X2 interface. The RNTP information has 1 bit indicating whether transmit power allocated according to each resource block (RB) exceeds a threshold. An RNTP value for a certain physical resource block (PRB) may be 0 or 1, wherein 0 means that the maximum transmit power of the PRB does not exceed the threshold, and 1 means that the maximum transmit power is not limited. The RNTP threshold is a parameter that determines a threshold for the above-described maximum transmit power.
The cells may know information indicating how much power is allocated to a specific RB in a neighboring cell by receiving the RNTP value of the neighboring cell, and may predict a level of interference to the RB. Since an RB has low interference if the neighboring cell allocates low power to the RB, a cell allocates high power to the RB for cell-edge users and the data transmission rate of the cell-edge users is capable of being increased.
On the other hand, since the RB has high interference if the neighboring cell allocates high power to the RB, the cell may allocate the RB for a user close to its own cell (that is, a user who is less affected by inter-cell interference) and power may be controlled to be allocated at a low level for the neighboring cell.
Likewise, for interference control of an LTE uplink (UL), eNBs exchange an overload indicator (OI) indicating an RB-specific uplink interference level. A UL OI value for a certain PRB may have a value of one of high interference, medium interference, and low interference, wherein the high interference indicates that the neighboring-cell interference of the PRB is high, the low interference indicates that the neighboring-cell interference of the PRB is low, and the medium interference indicates that the neighboring-cell interference of the PRB is medium.
For the UL interference control, the eNBs exchange a high interference indicator (HII) indicating an RB-specific UL interference level. A UL HII value for a certain PRB may be 0 or 1, wherein 0 indicates low interference sensitivity and 1 indicates high interference sensitivity.
Using the OI or HII indicating the above-described UL interference level, the cell allows the cell-edge users to use high power for a specific RB. That is, when high inter-cell interference is generated from the RB in the cell, the neighboring cell is induced to allocate the RB to a user close to its own cell (that is, a user who is less affected by inter-cell interference).
In the LTE of the related art as described above, ICIC information is exchanged between the eNBs through the X2 interference. FIG. 1 shows a method of exchanging ICIC information in the related art. In the LTE of the related art, ICIC considering a signaling amount and signaling delay secured by the X2 interface is performed in a semi-static scheme (a scheme in which coordination is performed in a unit of several seconds, not in real time).
However, if the X2 interface, which supports signaling between the eNBs, does not exist, or if it is difficult to sufficiently secure signaling delay performance even when the X2 interface exists, a method capable of performing ICIC without the X2 interface is necessary. Since it is difficult to effectively apply the semi-static scheme in which coordination is performed in a unit of several seconds in an environment such as a home eNB-femto-cell where inter-cell interference is rapidly varied, a dynamic interference coordination scheme is necessary.